This invention relates to a method for converting polycrystalline material to macrocrystalline material in ribbon form and more particularly to a method for ribbon-to-ribbon conversion utilizing a shaped molten zone.
In the manufacture of semiconductor devices such as transistors, diodes, integrated circuits, photovoltaic cells, and the like, the semiconductor industry uses large quantities of semiconductor material in the form of thin wafers or sheets. It has been conventional to produce these semiconductor sheets by first growing a single crystal semiconductor ingot, sawing the ingot into a plurality of thin sheets, and then lapping and polishing the sheets to the desired thickness and surface finish. While this process has proved satisfactory for most semiconductor devices, it is too expensive for some large area semiconductor devices, and especially for large area photovoltaic devices or solar cells. In fact, in order that photovoltaic devices become a viable alternate energy source, a significant reduction in the cost of the semiconductor starting material is essential.
Other techniques have been developed for the production of thin sheets of semiconductor material suitable for the production of solar cells. One of these is the so called ribbon-to-ribbon (RTR) conversion process which is expected to yield high quality substrates for use in solar cell fabrication as well as for use in the production of a variety of other semiconductor products. The RTR process uses a scanned beam of energy impinging on a polycrystalline ribbon to locally melt the ribbon and to induce crystal growth as the ribbon is translated past the energy beam. Thus a polycrystalline ribbon is transformed into a macrocrystalline ribbon by the process of localized melting and then recrystallization. A macrocrystalline structure is one in which the crystals are of sufficiently large size to permit efficient semiconductor action; a monocrystalline ribbon wherein the ribbon is but a single crystalline body is therefore encompassed within the term macrocrystalline. In this context, the word "ribbon" implies a strip having a width much greater than its thickness. Typical dimensions might be a length of up to several meters, a width of ten to one hundred millimeters, and a thickness of fifty to two hundred and fifty micrometers.
Although the RTR process has proved to be viable for the production of macrocrystalline substrates, the process heretofore practiced has not been capable of sufficiently rapid conversion to take full advantage of its economic potentiality. The limited conversion velocity results from difficulties with controlling the amount of molten material, removing the heat of fusion from the growing interface, and establishing the correct temperature profile in the cooling region to limit stress generation during cooldown.
Accordingly, a need existed for a method which would allow the ribbon-to-ribbon process to proceed at a more rapid rate than heretofore possible and would therefore make the RTR process economically feasible.
It is therefore an object of this invention to provide an improved process for the ribbon-to-ribbon conversion of polycrystalline material to macrocrystalline material.
It is a further object of this invention to provide an improved ribbon-to-ribbon process which improves the thermal gradient along the ribbon and reduces stress in the converted ribbon.